The present invention relates to a reflection liquid crystal display (LCD) using cholesteric liquid crystal, chiral nematic liquid crystal or liquid crystal in which these liquid crystals are blended, or holographic polymer dispersed liquid crystal.
It is strongly desired that a liquid crystal display having less power consumption is developed as a liquid crystal display (LCD) that is used for a portable telephone and a portable information terminal, whose technology has been remarkably progressed. Therefore, a display panel of a reflection liquid crystal display not requiring any back light has been actively developed. A reflection liquid crystal display of a TN system or STN system in which two polarization plates are used has been employed for watches and electronic calculators since previously. Also, a reflection liquid crystal display of a TFT system has been developed, in which the STN system and TN system that has reduced the number of polarization plates to one by disposing a reflection electrode which suppresses optical absorption by polarization plates.
Further, an STN birefringence ECB system LCD for color display, etc., has been developed, which does not employ a color filter having large optical absorption. Still further, as those in which no polarization plate and no color filter are used, a liquid crystal display has been developed, in which a guest host (GH), a holographic polymer dispersed liquid crystal (HPDLC), cholesteric liquid crystal and chiral nematic liquid crystal are used.
A reflection liquid crystal display, in which cholesteric liquid crystal, chiral nematic liquid crystal, or blended liquid crystal of these types of liquid crystal, or holographic polymer dispersed liquid crystal is used, utilizes Bragg""s reflection depending on a cyclic structure of a refractive index of liquid crystal layers.
Wavelength xcex of the above-described Bragg""s reflection is expressed by:
xcex=nxc2x7p 
where n is a mean refractive index of a liquid crystal layer, and p is a cycle of a refractive index.
Therefore, it is possible to produce a color LCD by stacking liquid crystal display panels having three reflection colors of RGB.
Also, in the present specification, a section of liquid crystal corresponding to respective pixels is defined to be a xe2x80x9cliquid crystal cellxe2x80x9d, a liquid crystal portion that expresses a picture brought about by aggregation of pixels displayed in the corresponding liquid crystal cells is defined to be a xe2x80x9cliquid crystal display panelxe2x80x9d (including a transparent substrate which is an obvious structure), and those including the corresponding liquid crystal panel, its drive circuit, and its control circuit are defined to be a xe2x80x9cliquid crystal display (LCD)xe2x80x9d.
However, in order for the color LCD to use the above-described three liquid crystal display panels, production costs are made remarkably expensive. Recently, there is a use for a black and white display in an electronic book, etc. However, as regards a liquid crystal display panel that utilizes Bragg""s reflection, it is based on a reflection display principle, in which display panels, respectively, reflecting light of wavelengths being in a relationship of two or more complementary colors are stacked to display in terms of black and white. Therefore, it is very difficult to display black and white by a single panel. As a trial, a liquid crystal display using the following liquid crystal is taken into consideration.
(1) Reflection wavelength band width xcex94xcex is expressed by
xcex94xcex=xcex94nxc2x7p 
where xcex94n is a difference in the refractive index, and p is a cycle of the refractive index.
Black and white display can be achieved by making the reflection wavelength band width large by using two or more liquid crystals whose refractive index difference xcex94n is increased. However, the display image becomes only a white reflection color, which is not satisfactory as a black and white display image.
(2) As regards cholesteric liquid crystal, chiral nematic liquid crystal or a mixture of these types of liquid crystal, polymer networks are formed in the liquid crystal, and helical axes of liquid crystal particles are disordered, whereby black and white display is achieved by widening the reflection wavelength band width. That is, there is a PSCT (Polymer Stabilized Cholesteric Texture).
But, in a liquid crystal display employing the liquid crystal referred to in (1) or (2) described above, it is very difficult to obtain a complete black and white display image. Also, in the three-layered type full color LCD, it is very expensive to obtain a complete black and white display image.
An example was reported, which showed a liquid crystal display capable of displaying four colors consisting of white, black, yellow and blue by stacking a liquid crystal display panel that reflects yellow and a liquid crystal display panel that reflects blue, which are in a relationship of complementary colors. However, since it becomes necessary to provide two liquid crystal display panels and a voltage drive circuit for the respective liquid crystals, the display becomes expensive in comparison with the number of colors that can be displayed.
An object of the present invention is to provide an inexpensive lamination type reflection liquid crystal display, which is obtained by stacking two liquid crystal display panels, and a method for producing the same.
The above-described object of the present invention can be solved by the following inventions (1), (2) and (3).
(1) A reflection liquid crystal display including two liquid crystal layers having selected reflection wavelengths differing from each other, and two pairs of liquid crystal display panels provided with a pair of transparent electrodes between which the respective liquid crystal layers are placed, and being obtained by stacking the two pairs of liquid crystal display panels; wherein voltage supply between a pair of electrodes of one liquid crystal display panel and voltage supply between a pair of electrodes of the other liquid crystal display panel are composed of a common voltage drive circuit.
It is preferable that the selected reflection wavelengths of the two liquid crystal layers in the reflection liquid crystal display according to the present invention are made in the relationship of complementary colors, and the color tone of the reflection light is made into black and white colors. In addition thereto, it is possible to obtain reflection light of a desirable color tone other than black and white by adequately selecting the selected reflection wavelengths of two liquid crystal layers.
Also, the liquid crystal layers can employ any one of cholesteric liquid crystal, chiral nematic liquid crystal, mixture of them and holographic polymer dispersion liquid crystal.
Chiral nematic liquid crystal may be independently employed as the above-described liquid crystal layer, or a mixture containing chiral nematic liquid crystal may be used. Also, a composite material including chiral nematic liquid crystal and other materials may be used as a liquid crystal layer. It is preferable that, as the chiral nematic liquid crystal, a compound showing a cholesteric phase as a single body, for example, a liquid crystal compound such as cholesteryl nanoate having an asymmetric center, etc., or that showing a cholesteric phase by blending a chiral agent to nematic liquid crystal, is used.
Also, a biphenyl-based, phenylcyclohexane-based, cyclohexyl cychlohexane-based, or pyrimidine-based liquid crystal compound may be independently employed, or nematic liquid crystal composed of a mixture of these liquid crystal compounds may be employed.
As the above-described chiral agent, a compound showing a cholesteric phase as a single body or an optically-activated organic compound showing a cholesteric phase by being blended with nematic liquid crystal although not showing any cholesteric phase as a single body, etc., may be used. As the optically-activated chiral agent, there is a compound similar to nematic liquid crystal, etc., such as an optically-activated ester derivative, an optically-activated cyanobiphenyl derivative, or an optically-activated bisphenol derivative.
Also, low-molecular weight liquid crystal or polymer weight liquid crystal may be used as the chiral nematic liquid crystal. In addition, a mixture which the former two are blended together, may be used. Where a sufficient self-holding property cannot be obtained in a case where the chiral nematic liquid crystal is independent, various types of solid grains are blended, and chiral nematic liquid crystal may be used as a surrounding spacer. As the spacer material, a rod-shaped or spherically-shaped glass substrate and plastic, or a columnar solid may be used.
A holographic polymer dispersion liquid crystal may be used as the above-described liquid crystal layer.
Also, by adjusting the fluctuation characteristics of a reflection ratio with respect to a drive voltage supplied between a pair of electrodes of the above-described two liquid crystal layers so that the characteristics become substantially identical to each other or almost the same, two sets of liquid crystal display panels may be stacked.
By varying the thickness of mutual liquid crystal layers, a type of nematic liquid crystal and chiral agent, and a blending ratio of the nematic liquid crystal and chiral agent, it is possible to make the drive characteristics of two liquid crystal layers, that is, the reflection characteristics with respect to a voltage supplied between a segment electrode and a common electrode of a liquid crystal display panel identical to each other, or almost the same.
The thickness of a liquid crystal layer having a larger rise voltage in the fluctuation characteristics of the above-described reflection ratio when the thickness of the two liquid crystal layers is made the same is adjusted to be thinner than that of the other liquid crystal layer having a lower rise voltage in the fluctuation characteristics of the above-described reflection ratio, whereby the same substance may be used for liquid crystal materials and a chiral agent, which constitute the two liquid crystal layers. Therefore, a liquid crystal insertion facility, etc., can be commonly used for two liquid crystal display panels, and there is an advantage in improving production efficiency of LCDs.
According to the method for adjusting the fluctuation characteristics of a reflection ratio with respect to a drive voltage of the above-described two liquid crystal layers by varying the type of the above-described nematic liquid crystal and chiral agent and/or its mixture ratio, since it is possible to keep constant the thickness of the respective liquid crystal cells in a liquid crystal display panel, into which liquid crystal is filled, the relationship with respect to complementary colors of the two liquid crystal display panels can be finely adjusted, wherein it is possible to minutely adjust the colors of the liquid crystal display image in a wide range from white to gray.
Also, the fluctuation characteristics of the reflection ratio with respect to the above-described drive voltage can be adjusted by providing a fixed or variable resistor in a voltage drive circuit of a liquid crystal layer having a smaller rise voltage in the above-described fluctuation characteristics of the reflection ratio. According to the adjusting method employing the variable resistor, since an external resistor electrically adjusts the fluctuation characteristics, there is a shortcoming in view of additional need of electric components. However, where there is unevenness when producing two liquid crystal display panels, the allowance thereof can be increased. Further, if a resistor element is a variable resistor element, it is possible to minutely adjust the characteristics of the reflection ratio to voltage in each of the liquid crystal displays.
(2) A reflection liquid crystal display including two liquid crystal layers having selected reflection wavelengths differing from each other; a pair of transparent electrodes consisting of segment electrodes and common electrodes between which the respective liquid crystal layers are placed; and two sets of liquid crystal display panels provided with a pair of transparent substrates between which the pair of transparent electrodes are placed; and being obtained by stacking respective segment electrodes and common electrodes of the two sets of liquid crystal display panels so as to be disposed at positions parallel to each other; wherein the reflection liquid crystal layer further comprises an electrical connecting means for connecting the segment electrodes of the respective liquid crystal display panels to each other; an electrical connecting means for connecting the common electrodes of the respective liquid crystal display panels to each other; and a voltage drive circuit of a single system, which supplies a voltage between the respective segment electrodes and common electrodes via the two electrical connecting means.
A flexible material, for example, FPC (Flexible Printed Circuit) may be used as the electrical connecting means.
Also, an overlapping plane of adjacent transparent substrates of the two sets of liquid crystal display panels are adhered to each other with an adhesive agent whose refractive index is almost the same as the refraction index of the transparent substrate, wherein a reflection loss on the overlapping plane of the above-described transparent substrates is reduced, and it is possible to increase the amount of light entering the liquid crystal cells in the second liquid crystal display panel when being observed from the light incident side, whereby image display having a desired color tone and a large contrast can be obtained.
In addition, by making the fluctuation characteristics of a reflection ratio with respect to a drive voltage identical to each other or almost the same on liquid crystal display by the construction of the present invention (2), it is possible to commonly use a voltage drive circuit that drives the respective segment electrodes and common electrodes of the two sets of liquid crystal display panels.
(3) A method for producing a reflection liquid crystal display, comprising the steps of: electrically connecting segment electrodes and common electrodes of the respective liquid crystal display panels, by flexible conductive materials, the respective liquid crystal display panels being composed of a first liquid crystal display panel in which the first liquid crystal is sealed between a pair of transparent substrates each being provided with segment electrodes and common electrodes, and a second liquid crystal display panel in which the second liquid crystal having a different selected reflection wavelength from the first liquid crystal layer is sealed between a pair of transparent substrates being provided with segment electrodes and common electrodes; folding the two liquid crystal panels connected, at the position of the flexible conductive material and making the same into a stacked body of two liquid crystal display panels; and electrically connecting the common electrodes and segment electrodes of the two liquid crystal display panels to each other.
Herein, the first liquid crystal display panel is produced by disposing the first transparent substrate having segment electrodes provided therein and the second transparent substrate having common electrodes provided therein so as to cause both of the electrodes to be opposed to each other and sealing the first liquid crystal between the transparent substrates, and the second liquid crystal display panel is produced by disposing the third transparent substrate having segment electrodes provided therein and the fourth transparent substrate having common electrodes provided therein so as to cause both of the electrodes to be opposed to each other and sealing the second liquid crystal having a different selected reflection wavelength from the first liquid crystal layer between the transparent substrates.
A description is given of a construction of a liquid crystal display panel according to the invention with reference to FIG. 1, using a liquid crystal display employing a cholesteric liquid crystal as an example. FIG. 1 is a sectional view of a liquid crystal display panel of a cholesteric liquid crystal display and shows a channel of external light incident into the liquid crystal display panel.
Cholesteric liquid crystal layers 3 and 3xe2x80x2 are, respectively, provided between the surface side glass substrate 1 and the rear side glass substrate 2, and between the surface side glass substrate 1xe2x80x2 and the rear side glass substrate 2xe2x80x2. Top coat layers 4, 4 and alignment layers 4xe2x80x2, 4xe2x80x2 composed of a polyimide macromolecular film and ITO layers 5, 5 and ITO layers 5xe2x80x2, 5xe2x80x2, which become electrodes, are, respectively, provided on both sides of the respective liquid crystal layers 3 and 3xe2x80x2. Further, the rear side glass substrate 2 and the surface side glass substrate 1xe2x80x2 are connected to each other by an adhesive agent 7. Also, a black layer 6 is provided on the rear side of the rear side glass substrate 2xe2x80x2, wherein black paint is coated thereon.
It is assumed that a combination consisting of a surface side glass substrate 1, rear side glass substrate 2, cholesteric liquid crystal layer 3, alignment layers 4, 4, and ITO layers 5, 5 is a liquid crystal display panel. Therefore, FIG. 1 shows a state where two sets of liquid crystal display panels 10 and 20 are stacked. A liquid crystal display according to the invention is such that a circuit, which supplies a current to electrodes 5a, 5axe2x80x2, 5b and 5bxe2x80x2 of the liquid crystal display panels 10 and 20 and controls the same, is added thereto.
In FIG. 1, illumination light I0 passes through the surface side glass substrate 1 and is made incident into the cholesteric liquid crystal layer 3. The cholesteric liquid crystal layer 3 has a structure in which liquid crystal particles are twisted, and the center axis of the twisting is called a xe2x80x9chelical axisxe2x80x9d. Bragg""s reflection of visible light occurs where the spiral pitch is in a range from 0.25 xcexcm to 0.46 xcexcm. Also, the liquid crystal layer 3 has a feature called bistability. A case where the helical axis of the cholesteric liquid crystal layer 3 is in an aligned state close to perpendicularity with respect to the glass substrates (surface side glass substrate 1 and rear side glass substrate 2) is called a planar texture, and a case where the helical axis is in an aligned state almost parallel to the surface of the glass substrates (surface side glass substrate 1 and rear side glass substrate 2) is called a focal conic texture. The two textures are stored in memory when no voltage is supplied.
In the planar texture, reflected illumination light I0 is reflected to the incident direction. In the focal conic texture, the reflected illumination light I0 advances in the direction of the rear side glass substrate 2. The illumination light I0 that has passed through the liquid crystal display panel 10 (hereinafter, this may be called a xe2x80x9cblue panel 10xe2x80x9d) that reflects blue light is made incident into the liquid crystal display panel 20 (hereinafter, this may be called a xe2x80x9cyellow panel 20xe2x80x9d) that reflects yellow light, and light that is reflected by the liquid crystal layer 3 of the planar texture of the yellow panel 20 is reflected in the incident direction.
On the other hand, in the focal conic texture of the blue panel 10, the illumination light I0 that is transmitted in the focal conic texture of the blue panel 10 advances in the direction of the rear side glass substrate 2xe2x80x2 of the yellow panel 20 and is absorbed by the rear side black layer (light absorbing layer) 6 of the rear side glass substrate 2xe2x80x2, wherein it seems to be black. Since the reflected light of the blue panel 10 and the reflected light of the yellow panel 20 are in a relationship of complementary colors, the reflection color becomes white. Selecting the planar texture and focal conic texture of the liquid crystal layers 3 and 3xe2x80x2 enables black and white display.
A reflection liquid crystal display according to the present invention can provide black and white display equivalent to paper inexpensively.